1. Field of the Invention
This invention relates to improvements in and relating to magnetic resonance imaging (MRI) and other NMR diagnostic techniques and in particular to a method of and apparatus for the investigation of time variant electromagnetic events, in particular substantially repetitive events, and especially rapidly variant events such as for example neuronal activity, cardiac activity, etc.
2. Description of the Prior Art
MRI is a diagnostic technique which has rapidly gained favor with physicians but which has tended to be use primarily for generating images of anatomical structures rather than for dynamic imaging of bodily events. Recently however MRI has been used to image blood flow and to produce dynamic images of physical activity, e.g. the heart's pumping action.
Detection of electrical activity in the body is also of particular medical interest. Thus for example the study of cardiac electrical activity may greatly facilitate identification of cardiac dysfunction.
There is thus a need for imaging modalities capable of the spatial and temporal resolution necessary for such investigations.
In MRI the magnetic resonance (MR) signals from which the MR images are generated derive from non-zero spin nuclei (the "imaging nuclei", usually chosen to be protons) within the subject being imaged. The characteristics of the signals depend to a large extent on the electromagnetic environment experienced by the imaging nuclei and any time variance of the electric/magnetic fields experienced by the imaging nuclei due to a localised time variant event, such as evoked neural potentials or cardiac electrical activity, will result in a variation in the degree of polarization of the imaging nuclei and potentially in the magnitude of the detected MR signals.
However in MRI the signal strength of the detected MR signals (the free induction decay (FID) signals) is proportional to the difference between the ground and excited nuclear spin state populations for the imaging nuclei before the excitation/detection cycles of the image acquisition period. For signals to be obtained with reasonable signal:noise ratios there has to be a fairly long delay period between such excitation/detection cycles in order to allow the nuclear spin system to polarise adequately. These delay times are typically of the order of the spin-lattice relaxation time of the imaging nuclei, e.g. of the order of seconds, and as a result the time dependence of the effects of fast repetitive events such as evoked neural potentials, cardiac electrical activity and the like, on the polarization rate is averaged out.